Interpretive Summary: Graphene oxide nanoparticles (GONPs) are engineered nanomaterials (ENMs) which have potential application to a wide variety of consumer products including heavy metal detecting sensors, electrodes, and antibacterial paper. There is concern, however, that if these GONPs are released into the environment that they may bioaccumulate and have a toxic effect on living organisms. With the potential hazard from GONPs entering the environment and disrupting delicate ecosystems, this study set forth to investigate the fate and transport of GONPs in complex aquatic systems. Specifically, the goal of this study was investigate the effects of dissolved organic matter (DOM) and the presence of a model divalent cation (Ca+2) on the transport behavior of GONPs in groundwater environments. This study showed that the presence of dissolved organic matter resulted in more stable GONPs thereby increasing the capacity to travel longer distances in the subsurface and potentially end up disrupting ecosystems or end up in the food chain from bioaccumulation. The increased transport can also potentially facilitate contaminant transfer throughout the environment as toxic substances attach to the GONP surface. Conversely, in the absence of DOM and in the presence of divalent Ca+2 ions, GONPs become unstable and form larger aggregates (~8.7 µm) compared with monovalent salts at the same concentration. These large aggregates will potentially settle out into sediment beds of aqueous environments or be filtered out in soil and aquifer materials. This study provides new information on important geochemical factors affecting GONP transport in the subsurface.

Technical Abstract:
A transport study investigating the effects of natural organic matter (NOM) in the presence of monovalent (KCl) and divalent (CaCl2) salts was performed in a packed bed column. The electrophoretic mobility (EPM) and effective diameter of the graphene oxide nanoparticles (GONPs) were measured as a function of NOM concentration. Suwannee River Humic Acid (SRHA) was used as the model NOM and was characterized across an environmentally relevant concentration range of 0.1-10 mg/L. Results suggest that the transport of GONPs is increased in monovalent (KCl) and divalent (CaCl2) salts in the presence of NOM. For NOM concentrations = 1.0 mg/l typically found in surface water environments, steric stabilization occurred and resulted in more stable GONPs. Conversely, in the presence of NOM concentrations (0.1 mg/L) found in some groundwater environments, slightly larger GONP aggregates were formed when compared to conditions in the absence of NOM, suggesting that bridging is occurring. In the absence of NOM, the effect of divalent ions increased GONP retention during transport in a packed bed column at lower salt concentrations (10-3 M CaCl2) compared to that for monovalent salts as reported in previous work. This study confirms that these “carbonaceous-oxide” materials follow traditional theory for stability and transport. This is the first study that identifies sensitivity of this unique carbon material to valence and NOM during transport in porous media that represent surface and groundwater conditions.